PRELIMINARY STUDY RESULTS Rain versus Snow in the Sierra Nevada, CA: Comparing free-air observations of melting level with surface measurements Jessica Lundquist 1, Paul Neiman 3, Brooks Martner 2,3, Allen White 2,3, Dan Gottas 2,3, Marty Ralph 3 1 Civil and Env. Engineering, University of Washington, Seattle, Washington 2 Cooperative Institute for Research in Environmental Science, University of Colorado, Boulder, Colorado 3 NOAA Earth Sciences Research Laboratory, Physical Sciences Division, Boulder, Colorado
California Profiler Network Vertical Profilers detect the altitude at which falling snow melts and changes to rain. In California, this altitude varies from 1 to 3 km. American River 2005
The altitude of the rain-snow line determines how much basin area contributes to runoff & floods.
Issue of rain vs. snow will become more important with warming temperatures, particularly in the west. American River Basin Computed from UW s VIC model daily INPUTS (Bales et al, in press) What fraction of each year s precipitation historically fell on days with average temperatures just below freezing? Less vulnerable Rain vs Snow More vulnerable Detailed snow vs rain model (Dettinger et al., in prep)
Floods in the American River Basin come from warm storms with high snow lines. Accurate knowledge of the rain-snow line (which changes hourly!) helps flood forecasts. Photo by Bryan Patrick, Sacramento Bee
Test how well radar melting level represents what happens on the surface. 6) 2190 m 1) 1610 m 5) 2100 m 2) 1710 m 4) 2010 m 3) 1800 m 7) 2320 m SWE & depth SWE only 8) 2440 m
We need colocated precip and snow depth to tell rain from snow.
T>2ºC 24-30 Jan 2005 T<0ºC Grass Valley melt level Bodega Bay melt level snow rain
Because of limited surface precip data, we use temperature as an index of rain vs. snow. 1) On rainy days, what is the probability of snow accumulating or melting at each temperature? 2) How well does the radar-measured melting level correspond with a melting temperature at the surface?
Probability contours for SWE/ precip ratio at 10 AR snow pillows during 2001-2005 Below 0ºC, all precip falls as snow (with some undercatch and overcatch at gages) 1 2
Probability contours for SWE/ precip ratio at 10 AR snow pillows during 2001-2005 1 2 From 0 to 2ºC, an increasing portion of rain falls with the snow
Probability contours for SWE/ precip ratio at 10 AR snow pillows during 2001-2005 Above 2ºC, snow is likely to melt, but the distribution is broad and uncertainty is large! Amount of melt depends on turbulent energy transfer (winds). 1 2 2.6ºC = 50% chance snow gain, 50% snow loss
Free-air melting levels, Radar brightband heights, and the surface snowpack
Melting level averages 1.8ºC in free-air but varies diurnally at surface Bodega Bay melting level (red circle) Grass Valley melting level (blue triangle)
Height adjustment from radar brightband level to AR surface snow change (2.6 C) Chico Bodega Bay Grass Valley Chowchilla All have diurnal variations and large standard deviations. hour of day Stats from 2001 to 2005.
Height adjustment from radar brightband level to surface snow change (2.6 C) Chico is to the north, where temperatures are cooler, = lower brightband at level of warmer AR T s, and less adjustment. hour of day
Height adjustment from radar brightband level to surface snow change (2.6 C) Chowchilla is to the south = warmer T s, higher melting levels, = cooler AR T s and greater adjustments. hour of day
Height adjustment from radar brightband level to surface snow change (2.6 C) hour of day Bodega Bay is to the west, on the coast, = temperatures are not so influenced by radiation, = largest diurnal changes and variance.
Conclusions The altitude where rain changes to snow is crucial for flood forecasting. Profiling radars detect the rain-snow altitude in the free atmosphere. Surface temperatures provide current best estimate of how and when that altitude may differ on the surface. Correcting by profiler location, time of day, and surface stats can improve elevation estimate RMSE by 150 m = 7.5% contributing area = 12 m 3 /s (850 ft 3 /s) in a medium-large AR storm
18-24 March surface Warm rain on 22 March = loss of snow depth and SWE at Blue Canyon.
T>2ºC 18-24 March 2005 T<0ºC Bodega Bay melt level rain snow Grass Valley melt level
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